Cryogenic Cooling Pump and Method

ABSTRACT

A pump ( 10 ) for circulating a cryogenic coolant, comprising: a cylinder housing ( 14 ); a piston ( 16 ) received in said cylinder housing ( 14 ) for reciprocating in said cylinder housing ( 14 ) between a first death center position ( 18 ) and a second death center position ( 20 ), said first death center position ( 18 ) and said second death center position ( 20 ) defining a compression chamber ( 22 ) therebetween; an outlet ( 34 ) for said compression chamber ( 22 ); and an inlet ( 36 ) formed in a wall ( 24 ) of said cylinder housing ( 14 ) at a position between said first death center position ( 18 ) and said second death center position ( 20 ); wherein said inlet ( 36 ) is adapted to establish a fluid connection between said compression chamber ( 22 ) and a coolant reservoir ( 40; 76 ).

FIELD OF THE INVENTION

The invention relates to a piston pump and a system for circulating a cryogenic coolant, as well as a corresponding method for circulating a coolant by means of a pump, in particular for circulating small amounts of re-condensed cryogens in the ml/s range and below.

BACKGROUND OF THE INVENTION

Cryocoolers and other storage vessels for cryogenic coolants are widely used in research and industry to cool samples, sensors or superconducting magnets. The object to be cooled may be attached to the cold head of the cryocooler, or may be placed into the condensate storage vessel of the cryocooler. Conventional cryocoolers, such as Gifford-McMahon (GM) cryocoolers or pulse tube cryocoolers, are often multi-stage cryocoolers, wherein a pre-stage or first stage provides the cooling for a thermal shield that surrounds the cold head, the condensate storage vessel, and/or the object to be cooled. The second stage supplies the cooling power at the lowermost temperature. The entire configuration, comprising the cryocooler, the object to be cooled, and possibly also the thermal shield may be placed in a cryostat. Such configurations are well-known in the art, and are suitable for applications in which the object to be cooled is an isolated object or is readily accessible for contact cooling. Oftentimes, though, the object to be cooled is integrated into a larger unit and cannot be placed in a cryostat, or cannot be directly connected to the cold head of a cryocooler. The unit may be located remotely or in an environment that is not directly accessible. The location of the object may not provide sufficient space for installing a cryocooler in close proximity. For instance, the object to be cooled may be a sensor which is integrated in a particle detector and placed at a location that may be exposed to high radiation levels, and thus cannot be brought into direct contact with the cold head of a cryocooler. What is needed is a compact and robust system that is capable of providing a constant flow of a cryogenic liquid or gas to a remote load that is not directly accessible.

SUMMARY OF THE INVENTION

This objective is achieved by means of a pump and pumping method with the features of independent claims 1 and 13, respectively. The dependent claims relate to preferred embodiments.

A pump for circulating a cryogenic coolant according to the present invention comprises a cylinder housing, and a piston received in said cylinder housing for reciprocating in said cylinder housing between a first death center position and a second death center position, said first death center position and said second death center position defining a compression chamber therebetween, and an outlet for said compression chamber. An inlet is formed in a wall of said cylinder housing at a position between said first death center position and said second death center position, wherein said inlet is adapted to establish a fluid connection between said compression chamber and a coolant reservoir.

The piston pump according to the present invention allows to provide an intermittent or continuous flow of coolant from said coolant reservoir through said inlet into said compression chamber, and further onwards through said outlet valve to the object to be cooled. Since the inlet is formed between the first death center position and the second death center position, the piston will periodically and alternately cover and open the inlet, and thereby doubles as an inlet valve. This allows for a simple, reliable and cost-efficient means of providing a coolant circulation flow. As a further advantage, the pump according to the present invention not only allows circulating the coolant to a remote load, but at the same time provides cooling effects. During the intake stroke of the pumping cycle, when the volume of the compression chamber increases and the piston opens or exposes the inlet, coolant will be sucked from the coolant reservoir and will expand into the compression chamber. The expansion provides for an additional cooling.

The compression chamber may be defined in said cylinder housing. The compression chamber may be confined by part of a sidewall and/or an end wall of said cylinder housing and an end surface of a piston when positioned at said first death center position and said second death center position, respectively. In other words, the compression chamber may comprise the volume swept by said piston between said first death center position and said second death center position.

The first death center position may correspond to a position at which said piston fully occupies said compression chamber. The second death center position may correspond to a position at which said piston is fully retracted from said compression chamber.

Said coolant reservoir may be a reservoir from which said coolant may be transferred to said compression chamber. In particular, said coolant reservoir may be a reservoir that is located externally to said pump.

In a preferred embodiment, said inlet is covered by said piston, in particular sealed by said piston, when said piston is at said first death center position and is opened when said piston moves from said first death center position towards said second death center position.

The pump may further comprise a fluid conduit in fluid communication with said inlet, said fluid conduit for establishing said fluid connection between said compression chamber and said coolant reservoir.

In a preferred embodiment, said inlet and/or said fluid conduit may be provided with an inlet valve for reversibly establishing said fluid connection between said compression chamber and said coolant reservoir. The inlet valve may provide a further means, besides the piston, for regulating a flow of coolant from said coolant reservoir into said compression chamber. Said inlet valve may be adapted to open if a pressure difference across said valve exceeds a predetermined pressure value, and in particular may be a one-way valve or check valve or non-return valve or spherical valve. The inlet valve may also be an electronically actuated valve, and may be controlled by a control unit.

The inlet, possibly enhanced by an inlet valve, may serve as a throttle to expand the liquid/vapour isenthalpically into the compression chamber, in accordance with the Joule-Thompson effect. The isenthalpic expansion leads to a temperature decrease of the liquid/vapour, thereby contributing to the desired cooling effect.

Said outlet may be formed in said cylinder housing. Said outlet may comprise an outlet valve formed in a wall of said cylinder housing, said outlet valve being in fluid communication with said compression chamber.

Said outlet valve may be formed at an end wall of said cylinder housing. Said outlet valve may be adapted to open if a pressure difference across said valve exceeds a predetermined pressure value, and in particular if a pressure in said compression chamber is higher, by a predetermined amount, than an ambient pressure beyond the valve. The outlet valve may be a one-way valve or a check valve or a spherical valve. The outlet valve may also be an electronically actuated valve, and may be controlled by said control unit.

The outlet valve may be adapted to open once the piston has been moved from said second death center position via said inlet and towards said first death center position to compress the coolant in the compression chamber. The coolant may then be expelled from the compression chamber through the outlet valve and towards the object to be cooled. After the coolant has been expelled from the compression chamber through the outlet valve, the piston may move from the first death center position towards the second death center position, thereby closing the outlet valve, opening or exposing the inlet and sucking in a new load of coolant, and the pumping cycle begins anew.

In a preferred embodiment, the pump comprises a driving unit for reciprocating said piston in said cylinder housing, wherein said driving unit is adapted to drive said piston from said first death center position towards said second death center position at a first velocity, and to drive said piston from said second death center position towards said first death center position at a second velocity, wherein said second velocity is lower than said first velocity.

By adjusting the first velocity higher than the second velocity, the coolant may expand relatively quickly during the intake phase to secure an isentropic or adiabetic expansion while being expelled relatively slowly. This may allow to provide additional local cooling effects. In addition, the relatively slow expulsion allows to achieve an approximately constant mass flow from the pump to the load, which is desirable in many applications.

In a preferred embodiment, said second velocity may be at least five times lower, preferably at least ten times lower, and particularly preferably at least twenty times lower than said first velocity. These velocity ratios may relate to an average velocity or to a peak velocity of said first and second velocity, respectively.

In a preferred embodiment, the pump comprises a driving unit for reciprocating said piston in said cylinder housing. Said driving unit may comprise a magnetic core integrated into said piston or mounted to said piston, and may further comprise a magnetic actuator located externally to said cylinder housing, wherein said magnetic actuator is adapted to generate a magnetic field for driving said piston by acting upon said magnetic core.

The driving unit as described above does not require any mechanical contact with the piston, and thereby allows to encapsulating the piston inside the housing for better thermal shielding and/or improved sealing.

In a preferred embodiment, the pump comprises a sealing element for sealing said piston against a sidewall of said cylinder housing. The sealing element may serve for sealing said compression chamber against the remaining interior of said (encapsulated) cylinder housing.

The pump may further comprise a bearing element for supporting said piston in said cylinder housing. A bearing element may be particularly advantageous to compensate for gravitational forces acting upon the piston when the piston is mounted horizontally or at an angle, and to avoid bending of the piston that may interfere with the sealing of the compression chamber.

Preferably, said bearing element comprises or integrates said sealing element.

In a preferred embodiment, an outer surface of said piston and/or an inner surface of said cylinder housing may be provided with grooves that serve as a dynamic sealing. Preferably, said grooves extend in a circumferential direction of said piston and/or said cylinder housing.

In a preferred embodiment, the pump comprises mounting means for mounting said pump to a cryocooler, wherein said coolant reservoir comprises a condensate storage vessel of said cryocooler and wherein said inlet is adapted to establish a fluid connection to said condensate storage vessel via a fluid conduit.

The inventors found that the pump according to the present invention can be conveniently combined with a commercially available cryocooler so to circulate the fluid provided in the condensate storage vessel of the cryocooler to a remote load, and possibly to further enhance the cooling function and provide additional cooling effect. The combination of a cryocooler with a pump according to the present invention permits the cooling of remote objects that do not admit contact cooling, thereby significantly enhancing the versatility of standard cryocoolers with minimum effort and only few modifications.

The invention hence also relates to a system for circulating a cryogenic coolant, comprising a cryocooler and a pump with some or all of the features as described above, wherein said coolant reservoir comprises a condensate storage vessel of said cryocooler and wherein said pump is mounted to said cryocooler, preferably detachably mounted to said cryocooler, and wherein said system comprises a fluid conduit for establishing a fluid connection between said inlet and said coolant reservoir.

In a preferred embodiment, said cylinder housing and/or said compression chamber may be thermally coupled to a cold head of said cryocooler, preferably at a lower end of said cylinder housing. By means of the thermal coupling, the cold head of the cryocooler may double to cool the cylinder housing and/or compression chamber.

The cryocooler may also serve to cool the sealing element and/or bearing element, thereby removing any excess heat that may be generated at the sealing element and/or bearing element due to the reciprocating movement of said piston in said cylinder housing.

In particular, the first stage part of the cryocooler may serve to remove said excess heat and may intercept heat conduction from those parts of the device that are at ambient temperature.

The sealing element and/or bearing element may be thermally coupled to said cryocooler via said cylinder housing.

Said cryocooler may be a multi-stage cryocooler, in particular a two-stage cryocooler. Said sealing element and/or said bearing element may then be thermally coupled to a first stage of said cryocooler, and/or said cylinder housing and/or said compression chamber may be thermally coupled to a second stage of said cryocooler. Said second stage of said cryocooler may comprise the cold head of said cryocooler for condensing said coolant.

The system may also comprise a thermal shield in thermal connection with said first stage of said cryocooler. Preferably, said thermal shield at least partially surrounds said pump and said compression chamber, and/or said coolant reservoir.

The system may also comprise an additional pump coupled to a first stage of said cryocooler.

Said additional pump may serve to provide coolant to said thermal shield, or may also provide coolant to the thermal shield of a load or any other component of the cooling circuit.

Said additional pump may be a pump according to the present invention with some or all of the features as described above, but may also be any conventional coolant pump.

The pump according to the present invention is not limited to complement or be integrated with cryocoolers, but can advantageously be employed with any device that holds a coolant in a coolant storage vessel.

The invention therefore also relates to a system for circulating a cryogenic coolant, comprising a coolant storage vessel and a pump with some or all of the features as described above, wherein said coolant storage vessel comprises said coolant reservoir and wherein said pump is mounted to said coolant storage vessel, preferably reversibly mounted to said coolant storage vessel, and wherein said system comprises a fluid conduit for establishing a fluid connection between said inlet and said coolant reservoir.

In a preferred embodiment, the system as described above may comprise first tubing means for establishing a fluid connection between said compression chamber and an object to be cooled via said outlet valve, and may further comprise second tubing means for establishing a fluid connection between said object to be cooled and said coolant reservoir. The first tubing means and second tubing means allow recirculation of the coolant from the compression chamber and via the object to be cooled to the coolant reservoir. This establishes a closed coolant circuit.

In a preferred embodiment, the system as described above may comprise a cryostat, wherein said pump and said cryocooler or said coolant storage vessel, respectively, and/or said first tubing means and/or said second tubing means may be at least partially housed in said cryostat.

The invention also relates to a method for circulating a coolant by means of a pump, said pump comprising a cylinder housing and a piston received in said cylinder housing for reciprocating in said cylinder housing between a first death center position and a second death center position, said first death center position and said second death center position defining a compression chamber therebetween, wherein the method comprises the steps of moving said piston from said first death center position to said second death center position, thereby exo posing an inlet formed in a wall of said cylinder housing and sucking said coolant from a coolant reservoir through said inlet into said compression chamber, moving said piston from said second death center position towards said first death center position past said inlet, thereby compressing said coolant in said compression chamber, and expelling said compressed coolant from said compression chamber through an outlet.

Said coolant may be a liquid coolant or a gaseous coolant, or may also be a multi-phase coolant, such as a two-phase coolant with a gaseous phase and a liquid phase.

Said step of moving said piston from said second death center position to said first death center position may comprise a step of covering said inlet and/or sealing said inlet, preferably covering and/or sealing said inlet by means of said piston. As described above, the piston may hence advantageously double as an inlet valve for said compression chamber.

In a preferred embodiment, said piston is moved from said first death center position to said second death center position at a first velocity, and is then moved from said second death center position to said first death center position at a second velocity, wherein said second velocity is higher than said first velocity. As described above, this allows temporary cooling power with a low pressure/temperature environment in which the stored fluid can expand and fill the volume.

In a preferred embodiment, said second velocity is at least five timeslower, preferably at least ten timeslower, and particularly preferably at least twenty times lower than said first velocity.

The pump may be a pump with some or all of the features as described above.

In particular, said pump may comprise a sealing element for sealing said piston against a sidewall of said cylinder housing, and/or may comprise a bearing element for supporting said piston in said cylinder housing.

The cooling reservoir may be a condensate storage vessel of a cryocooler.

In a preferred embodiment, the method comprises a step of cooling said sealing element and/or said bearing element by means of said cryocooler, preferably by means of a first stage of a multi-stage cryocooler.

The method according to the present invention may further comprise a step of providing said coolant expelled through said outlet to an object to be cooled. The method may further comprise a step of providing said coolant from said object to be cooled to said coolant reservoir, thereby closing the coolant circuit.

DESCRIPTION OF PREFERRED EMBODIMENTS

The features and numerous advantages of the piston pump and method according to the present invention can be best understood from a detailed description of the preferred embodiments in conjunction with the appended drawings, in which:

FIGS. 1 a and 1 b provide a cross-sectional side view and a front view, respectively, of a piston pump coupled to a cryocooler according to a first embodiment of the present invention;

FIG. 2 is a schematic drawing of a cooling circuit employing a cryocooler and piston pump according to the present invention;

FIG. 3 is a cross-sectional side view of a system according to a second embodiment of the present invention, comprising a cryocooler and two piston pumps according to the present invention, with a first piston pump for supplying a coolant flow to an object to be cooled and a second piston pump for cooling a thermal shield; and

FIG. 4 is a schematic sectional drawing of a piston pump according to a third embodiment of the present invention, when coupled to a coolant storage vessel.

The invention relates to a novel type of piston pump and circulation method that may be employed to circulate condensed cryogenic liquids, such as a pure liquid two-phase flow with liquid and vapour phases, or gases. The invention is particularly useful to circulate small amounts of re-condensed cryogens in the millilitre/second range and below. The invention also allows the expansion of part of the fluid, thereby providing a temporary cooling of the fluid that supports the cooling effect.

The inventors found that the novel pump can be conveniently and advantageously employed to complement commercially available cryocoolers to circulate a coolant to a remote load, thereby extending the capabilities of standard cryocoolers to remote non-contact cooling. The invention is particularly suitable for re-cooling and/or re-condensating cryogenic coolants such as liquid helium, but can also be applied at other temperature ranges.

FIGS. 1 a and 1 b show a sectional side view and a front view, respectively, of a piston pump 10 according to the present invention. The piston pump 10 is coupled to a cryocooler 12, which may be a Gifford-McMahon (GM) cryocooler, a pulse tube cryocooler, or any other kind of cryocooler. Any cooler that can be used to cool a coolant and/or a sample to cryogenic temperatures may be considered a cryocooler in the sense of the present invention, independently of the operating principle or the configuration details. As shown in FIG. 1 a and as will be described in further detail below, the pump 10 and the cryocooler 12 together provide an integrated system for circulating a cryogenic coolant to a remote load.

The piston pump 10 comprises a pump cylinder with a cylinder housing 14 in which a piston 16 is received for reciprocating in said cylinder housing 14 between a first death center position 18 and a second death center position 20. Said first death center position may correspond to a maximally compressed state of the fluid, in which the piston fully occupies the compression chamber. Said second death center position may correspond to a fully expanded state, when the piston has been fully retracted from the compression chamber. The first death center position 18 corresponds to a lowermost position of the piston 16, whereas the second death center position 20 corresponds to an uppermost position of the piston 16. A compression chamber 22 is defined in said cylinder housing 14 by a sidewall 24 of said cylinder housing 14, a bottom wall or end wall 26 of the cylinder housing 14 that corresponds or nearly corresponds to the first death center position 18, and a bottom or end surface of the piston 16 at the second death center position 20. In other words, the compression chamber 22 is defined by the swept volume between the first death center position 18 and the second death center position 20 of the piston 16. The piston travel or stroke may amount to a few centimetres, such as 2 cm to 5 cm or any other value, depending on the application.

A magnetic core, such as an iron core 28 may be integrated into the piston 16 at its upper end, and may cooperate with a coil 30 mounted externally to said cylinder housing 14, thereby providing a driving unit 32 for reciprocating the piston 16 between said first death center position 18 and said second death center position 20. The driving unit 32 allows to drive the piston 16 from outside the cylinder housing 14 without any mechanical contact, and hence the cylinder housing 14 may fully encapsulate the piston 16 to minimize the heat flow to the coolant and to provide for an efficient sealing. Any other type of linear inductor motor or step motor may also be employed.

An outlet valve, such as a spherical valve 34 is provided at the bottom of the compression chamber 22 in the end wall 26 of the cylinder housing 14. Preferably, said outlet valve is a low pressure drop non-return bulb valve with a strong sealing effect The outlet valve 34 will open automatically once the pressure with the compression chamber 22 exceeds the ambient pressure across the valve 34, which may depend on the overall pressure drop in the user-circuit that has to be overcome by the pump. This pressure drop should be as low as possible, preferably in the few mbars range.

The compression chamber 22 is further provided with an inlet 36. The inlet 36 comprises a bore hole, such as a circular hole, that may be formed in the sidewall 24 of the cylinder housing 14 at a height above said first death center position 18, but below said second death center position 20. The inlet 36 may be provided at a height ranging from 70 to 95% of the height of the compression chamber 22, with the height 0% corresponding to the first death center position 18 and the height 100% corresponding to the second death center position 20.

The dimensions of the cylinder housing 14 and of the piston 16 will in general be chosen such that the outer diameter of the piston 16 is only slightly smaller than the inner diameter of the cylinder housing 14, so that only a small gap is left between the inner sidewalls of the cylinder housing 14 and the piston 16. For instance, the diameter of the piston 16 may be by no more than 0.5 mm, and preferably by no more than 0.2 mm smaller than the inner diameter of the cylinder housing 14. Due to the position of the inlet 36 between the first death center position 18 and the second death center position 20, the piston 16 serves to cover and seal the inlet 36 against the compression chamber 22 when the piston 16 is at the first death center position 18 or between the first death center position 18 and the inlet 36. The piston 16 exposes the inlet 36 once it moves upwards beyond the inlet 36 and towards the second death center position 20, thereby establishing a fluid connection between the inlet 36 and the compression chamber 22. In other words, the piston 16 serves as a valve that will reversibly and alternately close and open the inlet 36, depending on the position of the piston 16 in the compression chamber 22.

The piston pump 10 is reversibly mounted to the flange 38 of the cryocooler 12, and extends alongside the cryocooler 12 such that the inlet 36 is connected via a conduit 44 to the condensate storage vessel 40 formed below the cold head 42. The conduit 44 can be flexibly connected to the inlet 36 to allow for thermalization. However, the conduit 44 and inlet 36 can also be integrated, without a flexible joint or connection. In this case, a flexible connection between the piston pump 10 and the cryocooler 12 could instead be provided at the warm end of the cryocooler, so to absorb any thermal movement at the warm end.

The operation and cooling cycle of the piston pump 10 will now be described with reference to FIGS. 1 a and 2. At the beginning of a cycle, the piston 16 is positioned at the first (lower) death center position 18. When the piston 16 is moved upwardly towards the second death center position 20 by means of the driving unit 32, the spherical valve 34 closes and the pressure in the compression chamber 22 drops. The dropping pressure leads to an expansion of the leftover coolant, and thereby to a concurrent drop in temperature. The pump 10 hence provides for additional cooling of the fluid. Once the piston 16 moves beyond the inlet 36, the inlet is exposed and cryogenic fluid/gas is sucked into the overcooled compression chamber 22 from a coolant storage reservoir in the condensate storage vessel 40 via the conduit 44. At this stage, the fluid fully or partially fills the compression chamber 22. With the next stroke in the opposite (downward) direction, the connection to the condensate storage vessel 40 is closed when the piston 16 moves downwardly from the second death center position 20 and beyond the inlet 36. The enclosed fluid is then compressed, which leads to the opening of the non-return valve 34 when the pressure is sufficiently high to overcome the counterpressure, and the cryogenic fluid is pushed out of the compression chamber 22 through the valve 34 and via the outlet tube 46 to a load 48 to be cooled. The load 48 may be a sensor or a superconducting magnet, or any other device that requires cryogenic cooling. The coolant will return from the load 48 towards the condensate storage vessel 40 via the return tube 50, and will condensate at the cold head 42 of the cryocooler 12 and accumulate in the condensate storage vessel 40. The piston 16 is now again at the first (lower) death center position, and the cycle may begin anew.

Due to the expansion stroke, the remaining gas in the cylinder is at lower temperature and pressure when compared to the fluid inside the storage vessel. Therefore, the fluid in the storage vessel is pushed into the cool volume of the cylinder. The system thereby comes back into the original position.

In order to provide for a fast expansion, the velocity of moving the piston 16 from the first death center position 18 towards the second death center position 20 may be chosen significantly larger than the velocity of moving the piston in the opposite direction from the second death center position 20 to the first death center position 18 while expelling the coolant. For instance, the velocity of moving the piston 16 from the first death center position 18 towards the second death center 20 may be chosen at least five times higher, preferably at least ten times higher and particularly at least twenty times higher than the velocity of moving the piston in the opposite direction from the second death center position 20 to the first death center position 18. These velocities may relate to peak velocities or average velocities along the way from the first death center position 18 to the second death center position 20 and vice-versa, respectively.

The piston-cylinder system is preferably placed vertically in operation, so that the piston 16 does not bend under its load and does not touch the inner surface of the cylinder housing 14. However, the pump 10 may also be placed horizontally if the application so demands. Gravitational forces or other loads acting upon the piston 16 may be absorbed by bearing elements 52, 52′, which support the piston 16 in the cylinder housing 14. The bearing element 52 is provided with a gap ring 54 that seals the piston 16 against the inner sidewall of the cylinder housing 14. Above the bearing element 52 and gap ring 54, the cylinder housing 14 may widen so to provide a larger gap between the outer sidewall of the piston 16 and the inner sidewall of the cylinder housing 14. For instance, the gap may amount to 0.1 mm to 0.2 mm above the bearing element 52 and gap ring 54, and may amount to 0.05 mm or lower below the bearing element 52 and gap ring 54.

An outer surface of the piston 16 and/or an inner sidewall surface of the cylinder housing 14 may be provided with a pattern of grooves that serve as a dynamic sealing. The dynamic sealing may allow to efficiently seal the compression chamber 22 and the piston 16 against the cylinder housing 14, without requiring mechanical contact between the outer sidewall of the piston 16 and the inner sidewall of the cylinder housing 14 and without inhibiting the relative movement of the cylinder housing 14 and the piston 16. For instance, the grooves of the dynamic sealing may extend in a direction perpendicular to the direction of movement of the piston 16 in the cylinder housing 14, and hence in a circumferential direction of the cylinder housing 14 and/or piston 16.

A further bearing element 52′ may be provided at an upper portion of the cylinder housing 14. The two bearing elements 52, 52′ cooperate to guide the piston 16 in a vertical direction. The bearing elements 52, 52′ and the seal 54 can be made from Teflon, a material with a very low coefficient of friction and sufficient flexibility at low temperatures. At the sliding faces between the bearing elements 52, 52′ and the piston 16, a thin metal cover 56 may be placed on the piston 16 to minimize the friction and avoid abrasion.

The bearing element 52 and the gap ring 54 may be thermally coupled to the cryocooler 12 via the cylinder housing 14 and a first flexible thermal connection 58 at an intercept 60. The thermal connector 58 may consist of a flexible material with high heat conductivity, such as copper or aluminium meshwork. This thermalization reduces the heat flow to the cold part of the pump 10 by conducting the heat produced at the bearing element 52 and the seal 54. For instance, the cryocooler 12 may be a two-stage cryocooler with a first stage 62 for providing a pre-cooling and a second stage supplying the cold head 42. The first stage 62 of the cryocooler 12 may then be employed to cool the bearing element 52 and the gap ring 54 at the intercept 60. A second flexible thermal connection 64 may be provided between the cold head 42 of the cryocooler 12 and the cylinder housing 14 at a lower end thereof.

As illustrated in the schematic drawings of FIG. 2, the first stage 62 of the two-stage cryocooler 12 may also cool a thermal shield 66 that surrounds and shields the cold head 42 of the cryocooler 12 and the lower part of the piston pump 10 (below the intercept 60), comprising the compression chamber 22 and outlet valve 34. The entire configuration comprising the piston pump 10 and the cryocooler 12 may additionally be housed in a cryostat 68.

Preferably, the cylinder housing 14 of the piston pump 10 is mounted to the cryocooler 12 only at the modified flange 38, to provide good sealing and avoid any thermal stress. The lower part of the piston pump 10 between the compression chamber 22 and the intercept 60 can be chosen rather long, preferably longer than 50% of the total length of the cylinder housing 14, so to minimize the heat flow from the bearing element 52 and gap ring 54 to the compression chamber 22. Preferably, the intercept 60 is placed at least 200 mm, and particularly preferably at least 300 mm above the compression chamber 22. The total length of the piston pump 10 as shown in FIGS. 1 a and 1 b may amount to 400 to 500 mm, but could also be smaller or larger, depending on the application and the size of the cryocooler that is used. The piston 16 itself can be made of a material with low heat conductivity, such as epoxy or G10.

FIG. 3 is a cross-sectional drawing of a circulation system according to a second embodiment of the present invention, and shows a piston pump 10 coupled to a two-stage cryocooler 12 as described above with reference to FIGS. 1 and 2. However in contrast to the configuration described previously with reference to FIG. 1, a second pump 70 is additionally mounted to the flange 38 of the cryocooler 12 to supply the thermal shield 66. The pump 70 can be connected to the first stage 62 of the cryocooler 12 and may circulate a coolant such as liquid helium at a temperature of about 50 K via an outlet tube 72 and a return tube 74 to cool the thermal shield 66 that surrounds the cold head 42, the condensate storage vessel 40 and the compression chamber 22. The second pump 70 can be a piston pump according to the present invention as described above with reference to FIGS. 1 and 2. However, any other pump could likewise be employed. With the cooling assembly shown in FIG. 3, a complete and compact cooling system can be provided that delivers a liquid helium flow at approximately 4.2 K from the cold head 42 and gaseous helium flow at about approximately 50 K that can be used for cooling the thermal shield 66.

Alternatively, the pump 70 can also be employed to cool a thermal shield at the remote load via the outlet tube 72 and return tube 74, or any other component in the cooling circuit.

Applications of the invention are not limited to cryocoolers. The inventive pump and circulation method may be employed to supply a coolant flow from any coolant storage vessel, as described in further detail with reference to FIG. 4. FIG. 4 shows a piston pump 10 as described previously with reference to FIGS. 1 to 3. However, instead of the cryocooler 12, the compression chamber 22 of the pump 10 is in fluid connection with a coolant storage vessel 76 via the inlet 36 and the conduit 44. The coolant storage vessel 76 is thermally coupled to the cylinder housing 14 of the piston pump 10 by means of a thermal connector 78 so to hold the lower part of the piston pump 10 and the compression chamber 22 at the temperature of the cryogenic liquid in the coolant storage vessel 76. The coolant storage vessel 76 and the lower part of the piston pump 10 are again surrounded by a thermal shield 66. Operation of the piston pump 10 and coolant cycle proceed just as described with reference to the previous embodiments, with the only exception that the coolant storage vessel 76 replaces the condensate storage vessel 40 of the cryocooler 12 for storing the coolant. The configuration shown in FIG. 4 may be employed as a supply pump in a cooling circuit or to decant cryogenic liquid.

The description of the preferred embodiments and the drawings merely serve to illustrate the invention and the numerous advantages it entails over the prior art, but should not be understood to imply any limitation. The scope of the invention shall be determined solely by means of the appended claims.

LIST OF REFERENCE SIGNS

-   10 piston pump -   12 cryocooler -   14 cylinder housing -   16 piston -   18 first death center position -   20 second death center position -   22 compression chamber -   24 sidewall of cylinder housing 14 -   26 bottom wall of cylinder housing 14 -   28 magnetic core of piston 16 -   30 coil -   32 driving unit, linear motor -   34 spherical valve -   36 inlet of compression chamber 22 -   38 flange of cryocooler 12 -   40 condensate storage vessel -   42 cold head -   44 conduit -   46 outlet tube -   48 load -   50 return tube -   52, 52′ bearing element -   54 seal/gap ring -   56 metal cover -   58 first flexible thermal connection -   60 intercept -   62 first stage of cryocooler 12 -   64 second flexible thermal connection -   66 thermal shield -   68 cryostat -   70 second pump -   72 outlet tube of second pump 70 -   74 return tube of second pump 70 -   76 coolant storage vessel -   78 thermal connector 

1. A pump for circulating a cryogenic coolant, comprising: a cylinder housing; a piston received in said cylinder housing for reciprocating in said cylinder housing between a first death center position and a second death center position, said first death center position and said second death center position defining a compression chamber therebetween; an outlet for said compression chamber; and an inlet formed in a wall of said cylinder housing at a position between said first death center position and said second death center position; wherein said inlet is adapted to establish a fluid connection between said compression chamber and a coolant reservoir.
 2. The pump according to claim 1, wherein said inlet is covered by said piston, when said piston is at said first death center position, and is opened when said piston moves from said first death center position towards said second death center position.
 3. The pump according to claim 1, comprising a fluid conduit in fluid communication with said inlet, said fluid conduit for establishing said fluid connection between said compression chamber and said coolant reservoir.
 4. The pump according to claim 1, comprising a driving unit for reciprocating said piston in said cylinder housing, wherein said driving unit is adapted to drive said piston from said first death center position towards said second death center position at a first velocity, and to drive said piston from said second death center position towards said first death center position at a second velocity, said second velocity preferably lower than said first velocity.
 5. The pump according to claim 1, wherein one or both of said outlet and said inlet is or are provided with valves.
 6. The pump according to claim 1, comprising a driving unit for reciprocating said piston in said cylinder housing, wherein said driving unit comprises a magnetic core integrated into said piston or mounted to said piston, and further comprises a magnetic actuator located externally to said cylinder housing, said magnetic actuator adapted to generate a magnetic field for driving said piston by acting upon said magnetic core.
 7. The pump according to claim 1, comprising a sealing element for sealing said piston against an inner sidewall of said cylinder housing.
 8. The pump according to claim 1, comprising mounting means for mounting said pump to a cryocooler, wherein said coolant reservoir comprises a condensate storage vessel of said cryocooler and wherein said inlet is adapted to establish a fluid connection to said condensate storage vessel via a fluid conduit.
 9. A system for circulating a cryogenic coolant, comprising a cryocooler and a pump, said pump comprising a cylinder housing; a piston received in said cylinder housing for reciprocating in said cylinder housing between a first death center position and a second death center position, said first death center position and said second death center position defining a compression chamber therebetween; an outlet for said compression chamber; and an inlet formed in a wall of said cylinder housing at a position between said first death center position and said second death center position; wherein said inlet is adapted to establish a fluid connection between said compression chamber and a coolant reservoir, wherein said coolant reservoir comprises a condensate storage vessel of said cryocooler and wherein said pump is mounted to said cryocooler, and wherein said system comprises a fluid conduit for establishing a fluid connection between said inlet and said condensate storage vessel.
 10. The system according to claim 9, wherein one or both of said cylinder housing and said compression chamber is or are thermally coupled to a cold head of said cryocooler at a lower end of said cylinder housing.
 11. The system according to claim 10, further comprising an additional pump coupled to a first stage of said cryocooler.
 12. A system for circulating a cryogenic coolant, comprising a coolant storage vessel and a pump, said pump comprising, a cylinder housing, a piston received in said cylinder housing for reciprocating in said cylinder housing between a first death center position and a second death center position, said first death center position and said second death center position defining a compression chamber therebetween, an outlet for said compression chamber, and an inlet formed in a wall of said cylinder housing at a position between said first death center position and said second death center position, wherein said inlet is adapted to establish a fluid connection between said compression chamber and a coolant reservoir, wherein said coolant storage vessel comprises said coolant reservoir and wherein said pump is mounted to said coolant storage vessel, and wherein said system comprises a fluid conduit for establishing the fluid connection between said inlet and said coolant reservoir.
 13. A method for circulating a coolant by means of a pump, said pump comprising a cylinder housing and a piston received in said cylinder housing for reciprocating in said cylinder housing between a first death center position and a second death center position, said first death center position and said second death center position defining a compression chamber therebetween, wherein said method comprising: moving said piston from said first death center position to said second death center position, thereby exposing an inlet formed in a wall of said cylinder housing and sucking said coolant from a coolant reservoir through said inlet into said compression chamber; moving said piston from said second death center position towards said first death center position past said inlet, thereby compressing said coolant in said compression chamber; and expelling said compressed coolant from said compression chamber through an outlet.
 14. The method according to claim 13, wherein said moving said piston from said second death center position to said first death center position comprises covering said inlet and/or sealing said inlet.
 15. The method according to claim 13, wherein said piston is moved from said first death center position to said second death center position at a first velocity, and wherein said piston is moved from said second death center position to said first death center position at a second velocity, wherein said second velocity is higher than said first velocity.
 16. The method according to claim 13, comprising providing said coolant expelled through said outlet to an object to be cooled.
 17. The pump according to claim 2, wherein said inlet is sealed by said piston, when said piston is at said first death center position, and is opened when said piston moves from said first death center position towards said second death center position.
 18. The system of claim 10, wherein said pump is detachably mounted to said cryocooler.
 19. The system of claim 14, wherein said pump is detachably mounted to said coolant storage vessel.
 20. The method of claim 17, wherein said covering and/or sealing comprises covering and/or sealing said inlet by means of said piston.
 21. The method of claim 20, further comprising providing said coolant from said object to be cooled to said coolant reservoir. 